Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related alloys are known to be excellent catalysts. When incorporated in electrodes of an electrochemical device such as a fuel cell, these materials function as electrocatalysts since they accelerate electrochemical reactions at electrode surfaces yet are not themselves consumed by the overall reaction. Although noble metals have been shown to be some of the best electrocatalysts, their successful implementation in commercially available energy conversion devices is hindered by their high cost and scarcity in combination with other factors such as a susceptibility to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and the relatively slow kinetics of the oxygen reduction reaction (ORR).
A variety of approaches have been employed in attempting to address these issues. One well-known approach involves increasing the overall surface area available for reaction by forming metal particles with nanometer-scale dimensions. However, a primary challenge with the use of nanoparticulate electrocatalysts is that these zero-dimensional (0D) morphologies possess proportionally higher numbers of defect sites, lattice boundaries, and low coordination atoms at their surfaces. Inherently, defect sites are substantially less active towards oxygen reduction reaction than defect-free crystal planes, largely because of differences in the local coordination geometry and surface energy, which can directly influence the interfacial interaction between the metal surface sites and the adsorbed oxygen species. In addition, defect sites are more readily passivated by the presence of adsorbed OH groups, which decrease the number of active metal surface sites and inhibit catalysis at potentials below 1.0 V. Not surprisingly, defect sites are also irreversibly oxidized more readily under operating potentials of 0.7 V, resulting in enhanced catalyst degradation, and thus, the 0D catalysts lack sufficient durability for long-term usage in practical, functional fuel cells.
Noble metal nanostructures possessing one-dimensional (1D) morphologies have been highlighted as the structural motif that may solve many of the inherent catalytic problems associated with the nanoparticulate catalysts. Specifically, 1D nanostructures are characterized by their uniquely anisotropic nature, which imparts advantageous structural and electronic factors in the catalytic reduction of oxygen. For instance, the structural anisotropy of highly crystalline (crystallized) 1D motifs results in the preferential exposure of low energy crystalline planes so as to minimize the surface energy of the system. In the case of platinum, the low energy (111) and (110) facets are most active toward oxygen reduction reaction in perchloric acid solution, thereby making these anisotropic structures uniquely advantageous as oxygen reduction reaction electrocatalysts. Similarly, the preferential display of smooth crystalline planes minimizes the number of undesirable low-coordination defect sites, which is expected to enhance both ORR activity and long-term durability. These factors are expected to culminate in the suppression of the cathodic overpotential by delaying surface passivation of adsorbed OH groups to higher potentials and thereby increasing the interfacial ORR kinetics. Despite these tangible gains, continued improvement is necessary in order to meet the 2015 U.S. Department of Energy (DOE) target for performance in an oxygen reduction environment under membrane electrode assembly (MEA) conditions, namely a platinum group metal mass activity of 0.44 A/mg (at 0.9 V).
A promising route toward improving noble metal, e.g., platinum (Pt), mass activity has been the development of core-shell nanoparticles in which a core particle is coated with a shell of a different material which functions as the electrocatalyst. The core is usually a low cost material which is easily fabricated whereas the shell comprises a more catalytically active noble metal. An example is provided by U.S. Pat. No. 6,670,301 to Adzic, et al. which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported by carbon (C) substrates. Another example is U.S. Pat. No. 7,691,780 to Adzic, et al. which discloses platinum- and platinum-alloy coated palladium and palladium alloy nanoparticles. Each of the aforementioned U.S. patents is incorporated by reference in its entirety as if fully set forth in this specification.
Typically, the synthesis of the electro catalyst nanostructures made with noble and non-noble metals, including the core particle of the core-shell nanoparticles can be achieved via a polyol process, photochemical routes, seeded growth or site-selective lithographic approaches. (Teng et al., J Phys. Chem. Lett. 112, 2008, 14696-14701; incorporated herein by reference in its entirety). However, these methods provide limited ability to finely control the size, morphology, phase-segregation, and position of the components in the crystal based nanostructures, partially reflecting the structural complexity imposed by the random nucleation of the metal component.
Solution-based synthesis with directing, dispersing and capping agents, e.g., phase-transfer, is an important synthetic strategy, which is widely applied to control morphology and/or spatial orientation of inorganic crystals. In particular, metallic nanoparticles of different shapes and sizes can be obtained via either a direct synthesis in the organic phase, or by transferring nanoparticles from the aqueous phase to the organic phase. The latter has the advantage of leveraging many existing methods for preparing metallic nanoparticles in the aqueous phase, and avoiding the use of expensive organometallic precursors. On the other hand, the low interfacial energies observed in non-polar organic solvents may enable better control during surface and solution processing. A detailed description of the available phase transfer procedures to generate metallic nanoparticles is provided in Sastry (Curr. Sci., 2003, 85, 1735-1745) and Medintz et al. (Nat. Mater., 2005, 4, 435-446). Each of the aforementioned publications is incorporated by reference in its entirety as if fully set forth in this specification.
For example, the Brust-Schiffrin method is the earliest solution based phase transfer approach for preparing stabilized metal nanoparticles, such as gold (Au). In this approach, the gold ions from an aqueous solution were first extracted to a hydrocarbon phase, e.g. toluene, using tetraoctylammonium bromide (TOAB) as the phase transfer agent. The transfer of gold ions was facilitated by the electrostatic attraction between the positively charged tetraoctylammonium anions and the negatively charged tetrachloroauric cations. Subsequent reduction reactions took place in the organic solution using an aqueous NaBH4 solution in the presence of an alkanethiol, yielding Au nanoparticles of ˜2.5 nm. (Yang et al., Chemical Society Review 40, 2011, 1672-1696; incorporated herein by reference in its entirety) In the phase-transfer approach, surfactants or other capping agents are added during synthesis to function as the directing agent(s) to influence the growth of the crystal into a desired motif by preferential adsorption to different crystal faces during the growth. The surfactants may also function as dispersing agent(s), e.g., a soft micellar template, to prevent particle agglomeration. Once the desired size and shape have been achieved, the crystal growth is stopped through rapid reduction. These solution chemistry methods have been used, for example, to make nanoparticles of palladium (Pd), gold (Au), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium (Os), rhenium (Re), nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), and combinations thereof.
Although, the solution based synthesis affords careful control of the size, shape, and composition that allows the properties of various noble metals, particularly palladium and platinum, to be varied, the method inherently deactivates the electrocatalytic functionality of the synthesized nanostructures. Thus, in order to utilize these nanoparticles as electrocatalysts they must be activated by efficiently removing capping agents and surfactants from the surface of the metallic nanostructures without causing increased nanoparticle thickness, breakage, or undesirable low coordination sites. Traditionally, removal of the organic material requires the use of additional washing and/or heating processes. However, even with the appropriate cleaning steps, a residual organic layer typically remains on the surfaces of the nanoparticles. Alternatively, the residual organic layer made from surfactants and capping agents can be removed through oxidation by acid wash or UV generated ozone treatment. The procedure, however, may require up to several hours of exposure, which inadvertently leads to unwanted oxidation of the nanoparticles themselves, thereby generating undesirable low coordination sites.
There is, therefore, a continuing need to develop methods for effectively activating nanostructure electrocatalysts necessary for large-scale and cost-effective processes suitable for commercial production and incorporation in conventional energy production devices without damaging the surface of the generated nanostructures.